The invention concerns a method of detecting the instantaneous position of a piston attached to a piston rod inside the cylinder of a dashpot in a motor vehicle. The cylinder can be magnetized. The dashpot has a protective cover coupled to the piston rod.
Dashpots of this type are employed to transfer vibrations from the surface of the road to the chassis by way of the wheel suspension. To ensure a safe and comfortable ride, such dashpots are often semi-actively or actively controlled. For such controls it is necessary to determine the various parameters of position and motion and to forward them to a processing circuit. There is an advantage to detecting the position of the piston in the cylinder. The detection results in a signal representing how far inside the cylinder the piston is. This signal can be used to regulate levels when necessary. It can also be differentiated to obtain the parameters of motion needed to control the level of vibration suppression.
German OS 3 538 349 discloses several devices for detecting the road clearance of a vehicle. One for example is a coil with primary and secondary windings mounted around the dashpot cylinder and supplied with electricity from a voltage-frequency generator. To determine the position of the piston, the magnetic flux generated by the coil is attenuated by the piston's magnetic material, and the induced currents debilitate the high-frequency signal at the ends of the coil. The drawback to this instrument is that both the piston and the electrically conductive piston rod attenuate the magnetic flux. Precise results can accordingly be attained only from forward-axle dashpots with strictly design-dictated slight variations in cross-section between the piston and the piston rod. To obtain signals that are reliable enough for processing, the cylinder must be of non-magnetic material. This sensor can accordingly be employed only with special dashpots.
Another problem is encountered in single-tube dashpots in particular. In the known sensor, the leads from the windings around the cylinder are subject to severe stress from the moving wheels while the vehicle is in operation. They are also exposed to dirt and damp.